1. Field of the Invention
The present invention relates to a retransmission control method and a retransmission control apparatus used in transmitting digital data sequences by radio by using orthogonal frequency division multiplexing (OFDM) signals.
2. Discussion of the Background
There is an increasing demand for indoor or outdoor radio data communication systems achieving high-speed data communications. In high-speed radio communication systems, radio signals disadvantageously travel to a receiver by more than one route because they are reflected by obstacles, such as buildings. This is referred to as “multipath interference”, resulting in significant degradation of receiving performance. It is therefore very important to decrease the multipath interference in the above type of system. A typical countermeasure being taken against this multipath interference is the use of an equalizer. However, this is not practical because the equalizer is too large for a high-speed radio communication system and seriously hampers the achievement of miniaturized and low-powered high-speed radio communication systems.
Thus, radio communication systems using the OFDM technique, which is robust to multipath interference, are being considered. In the OFDM technique, a multi-carrier transmission system is employed in which signals are transmitted while being superimposed on multiple carriers positioned at minimal orthogonal intervals. In this technique, influence from intersymbol interference caused by multipath distortion can be significantly suppressed, and thus, the quality of the receiving performance can be maintained. The use of the OFDM technique also significantly lowers the transmitting rate of each subcarrier and therefore makes it possible to modulate and demodulate multi-carrier signals by digital-signal batch processing (Fast Fourier Transformation (FFT) and Inverse Fast Fourier Transformation (IFFT)).
However, an OFDM signal waveform per unit time (OFDM symbol), which is also referred to as a time waveform, is like a noise. Thus, the amplitude fluctuation of a time waveform is greater in the multi-carrier transmitting method than in the single-carrier transmitting method. Accordingly, if the back-off amount of a transmission power amplifier is set to be small, nonlinear distortion by the power amplifier occurs, thus lowering the transmission quality, i.e., causing transmission errors.
On the other hand, if, considering a great level of amplitude fluctuation, the back-off amount of the transmission power amplifier is set to be large, transmission power cannot be amplified with high efficiency. A linearizer, which varies the back-off amount, may be used for making the power amplifier operable even with a small back-off amount. However, the use of a linearizer is not suitable because a linearizer is too large for a radio communication system, which hampers the achievement of a miniaturized, low-powered, and inexpensive radio communication system.
One of the solutions to the above problems may be to detect the peak power of each OFDM symbol so as to control the average power of OFDM symbols in accordance with the detected peak power. In this method, the time waveform of each OFDM symbol is normalized by the peak power so as to make the peak power of all the symbols uniform. This makes it possible to reduce the back-off amount of the transmission power amplifier.
According to this method, on the one hand, the transmission power amplifier can be operated with high efficiency, but on the other hand, the transmission quality varies according to the OFDM symbols. Therefore, the average power of OFDM symbols having a large amplitude fluctuation and a high level of amplitude is highly suppressed, thus degrading the transmission quality, i.e., encouraging transmission errors.
Therefore, in performing data communications between a radio base station and a radio terminal station, if transmission errors are detected, a request is made to retransmit the same data. However, since such transmission errors are caused by the time waveform of an OFDM symbol, errors may occur again to the retransmitted data, thereby causing a decrease in the throughput.
As discussed above, in the conventional OFDM transmitting method the transmission power amplifier can be operated with high efficiency without requiring a large-scale apparatus. In this method, however, the average power of OFDM symbols having a large amplitude fluctuation and a high level of amplitude is suppressed. As a consequence, the transmission quality is lowered, i.e., the transmission errors are encouraged. There is also a high possibility of causing errors in the data retransmitted in response to a retransmission request. This entails repeated transmission, thereby lowering the throughput.